Camera renders blurry image in darker part of scene
It seems that your focus point for the camera is attatched to some other object in the scene, so that when you get into the air duct, it is all blurry. There are several ways to fix this, the best one is to add an empty (Shift+A > Empty > Plain Axis). Name it Camera Focus or something descriptive so you can find it easily later.

Now select the camera and go to Camera Settings > Depth of Field and select your empty.



You can now animate the position of the empty, and the camera will automatically focus on that point, so you can have it positioned near where you want to focus while it is still in the room, and then animate it to move into the air duct.

If your preview image is still pixelated like that, try rendering at a higher sample count.